Some asymmetric ethylene-bridged metallocenes derived from group 4 metals and containing fluorenyl and indenyl fragments have already been proposed for the polymerization of olefins such as propylene (EP-A-0 754 698).
However, most of them lead to poor isotacticity. It is so that 1-(.eta..sup.5 -9 fluorenyl)-2-[.eta..sup.5 -1-(2-methyl)indenyl]ethane zirconium dichloride used in combination with aluminoxanes (MAO) produces polypropylene having a limited stereospecificity (about fifty mole % mm triads) and a rather low melting point. On the other hand, the 1-(.eta..sup.5 -9-fluorenyl)-2-[.eta..sup.5 -1-(4,7-dimethyl)indenyl]ethane zirconium chloride, when used in combination with MAO, produces a purely atactic polypropylene (mm triads about 25%) characterized by the absence of any melting point.
Ethylene-bridged indenyl fluorenyl metallocenes are generally produced by a route comprising the reaction of fluorenyllithium with ethylene oxide to form 9-hydroxyethylfluorene which in turn is reacted with 1 equivalent of trifluoromethanesulfonic acid anhydride to replace the hydroxyl group by a --OSO2CF3 group; the said sulfoderivative being finally reacted with 1,2 equivalent indenyllithium to produce the 1-(9-fluorenyl)-2-(1-indenyl)ethane metallocene precursor (B. Rieger et al., Organometallics, 1994, 13, 647-653).
This known synthesis route for ethylene-bridged indenyl fluorenyl metallocene precursors has the following drawbacks: it does require the use of excess of indene, the use of ethylene oxide and the fluorenyl intermediate (triflate) is unstable and thus not isolable.